As it is known, systems for harvesting energy from mechanical or environmental energy sources arouse considerable interest in a wide range of technological fields, for example in the field of portable or mobile electronic apparatuses.
Typically, energy-harvesting systems are designed to harvest and store energy generated by mechanical or environmental sources and to transfer it to a generic load of an electrical type, which may be supplied, or, in the case of an electric battery, recharged. Such systems may, for example, allow to provide portable electronic apparatuses without batteries or to considerably increase the lifetime of batteries of such portable electronic apparatuses.
In particular, an energy-harvesting solution that has been proposed envisages use of a MEMS piezoelectric device capable of converting into electrical energy the energy associated to mechanical vibrations (originating from the environment in which the device is operating or from bodies, with which the device is in contact).
The simplest solution of a MEMS piezoelectric device envisages use of a cantilever structure, which carries at a free end an inertial mass, or “proof mass”; an appropriate piezoelectric material, such as for example PZT (lead zirconate titanate), is placed on the cantilever beam.
In this above solution, mechanical vibrations cause movement of the proof mass and generation of kinetic energy, which is converted into potential elastic energy in the form of mechanical deformation of the cantilever beam and of the associated piezoelectric material.
In particular, the resulting oscillation of the cantilever beam causes tensile and compressive effects in the piezoelectric material and the resulting generation of an electric charge that may be detected at electrodes arranged in contact with the piezoelectric material. In other words, part of the potential elastic energy stored in the cantilever beam is converted into electrical energy.
The above solution has an important disadvantage linked to a very narrow operating frequency band in so far as the cantilever structures are generally designed for having a high quality factor Q. Even slight deviations from the natural resonance frequency of the mechanical structure thus cause a sharp decay of the energy that may be harvested.
However, mechanical vibrations in the environment generally have a wide frequency spectrum, with non-controllable values. It follows that the solution described previously is not typically capable of offering an adequate efficiency in terms of energy harvesting.
To overcome the above drawback, a further solution that has been proposed envisages use of a so-called “doubly clamped” structure, configured to generate stresses that are predominantly of a tensile type on corresponding piezoelectric elements. These devices show a behavior dominated by linear bending in the high-frequency region and with low oscillation amplitudes, and by markedly non-linear stretching in the low-frequency region and with high oscillation amplitudes.
As illustrated schematically in FIG. 1A, this solution envisages use of a pair of thin cantilever elements 1a, 1b, i.e. ones having a thickness t along a vertical axis z much lower than a corresponding main extension (or length) along a first horizontal axis x. The cantilever elements 1a, 1b are constrained at a first end to a fixed structure 2, and at a second end to a proof mass 4, which is directly coupled to the cantilever elements 1a, 1b and arranged centrally with respect to the same cantilever elements 1a, 1b. 
In particular, D1 and D2 in FIG. 1A designate the main extension of the proof mass 4 and the total extension of the structure (given by the sum of the lengths of the cantilever elements 1a, 1b and of the proof mass 4), along the first horizontal axis x.
As illustrated in FIG. 1B, a displacement δ along the vertical axis z of the proof mass 4 as a result of the mechanical vibrations causes tensile stresses of deformation on both of the cantilever elements 1a, 1b. In particular, the fact that the proof mass 4 is arranged at the center prevents lateral movements or rotations of the proof mass 4 and enables reduction of the natural oscillation frequency to values of hundreds of Hertz, which practically correspond to the typical values of the spectrum of environmental mechanical vibrations.
An example of MEMS piezoelectric device, in particular an energy-harvesting generator, that uses the doubly clamped solution discussed previously, is described in the document: Hajati Arman, Sang-Gook Kim, “Ultra-wide Bandwidth Piezoelectric Energy Harvesting” Applied Physics Letters 99.8 (2011): 083105, 2011 American Institute of Physics (incorporated by reference).
In brief, and as illustrated in FIG. 2A, the micromechanical structure of the MEMS piezoelectric device described in the above document and designated by 10 comprises a supporting body 11, of semiconductor material, in particular silicon, in which a cavity 12 is provided. A membrane 13 is arranged over the cavity 12 and carries at the center an inner proof mass 14, directly coupled to the membrane 13.
The inner proof mass 14 defines in the membrane 13, laterally with respect to the same proof mass 14, a first cantilever element 15a and a second cantilever element 15b, on which an appropriate piezoelectric material element 16, for example PZT, is placed, contacted by electrodes 17.
During the manufacturing process, the etching leading to formation of the cavity 12 also defines the geometry of the inner proof mass 14 and the dimensions of the cantilever elements 15a, 15b, which are thus determined directly by the size of the same inner proof mass 14.
In particular, once again D1 and D2 designate in FIG. 2A the extension of the inner proof mass 14 and, respectively, the main overall extension of the cantilever elements 15a, 15b and of the aforesaid inner proof mass 14.
The micromechanical structure 10 further comprises an outer proof mass 18, having an extension substantially corresponding to the aforesaid dimension D2, coupled, for example using bonding techniques, onto the membrane 13.
During operation, in the presence of environmental vibrations, the cantilever elements 15a, 15b undergo deformation as a result of the joint displacement of the inner and outer proof masses 14, 18, thus generating a corresponding electrical signal at the electrodes 17.
As illustrated in FIG. 2B (where, for reasons of simplicity, the outer proof masses 18 are not illustrated), a plurality of micromechanical structures 10 of the type described previously may advantageously be obtained starting from a same supporting body 11, in order to increase efficiency of generation of electrical energy.
It is noted that the solution described previously is, however, affected by certain important limitations. In particular, the electrical performance depends upon the geometry of the resulting proof mass.
However, it is not possible to increase the dimensions of the inner proof mass 14 beyond a certain threshold in so far as, in this case, the dimensions of the cantilever elements 15a, 15b (and of the corresponding piezoelectric material elements 16) would be excessively reduced; typically, in the micromechanical structure 10, dimension D1 may at the most be equal to one third of dimension D2.
Introduction of the outer proof mass 18 is consequently required to increase the performance of energy generation, starting from the detected mechanical vibrations.
However, coupling of this outer proof mass 18 complicates the manufacturing process (requiring in fact coupling between two distinct wafers of semiconductor material, the so-called “wafer-to-wafer bonding”) and leads to a non-negligible possibility of defectiveness following upon dicing of the same wafers.
Furthermore, the geometry of the piezoelectric structure is in this case fixed, being of the doubly clamped type based on the tensile deformation modes of the piezoelectric material, thus limiting the possibilities in designing the characteristics of the micromechanical structure.
There is accordingly a need in the art to overcome, at least in part, the problems that afflict MEMS piezoelectric devices of a known type, and in particular to provide a more efficient solution for harvesting of environmental energy.